Tactic - of a movement made by a whole organism, or by a freely motile part such as a gamete, in response to a directional stimulus which has a direct bearing on the direction of the elicited response.
See also: Phototaxis

Tannin - a heterogeneous group of astringent phenolic substances, widely distributed in plants, and commercially used in tanning, dyeing and preparation of ink.

Taproot - a primary root growing straight down from a plant, and larger or longer than the branching fibrous roots. Often a thick storage organ as in a carrot.

Thallus - a multicellular plant body, usually flat and not organized into roots, stems, leaves or vascular system.

Theophrastus - (ca. 372 - ca. 288 BC) - the most important and influential botanist in the ancient world. He created the first botanical garden in Athens and discovered the method for artifical pollination of date palms that is still used today. The 227 treatises attributed to Theophrastus include two botanical works:

Historia de Plantis (A History of Plants)

De Causis Plantarums (About the Reasons of Vegetable Growth)

These works are of a general nature but remained for a long time the authority in botanical literature.

Tissue - any part of an organism consisting of aggregated cells with a similar structure and function.

Tomentum - a layer of matted woolly threads on the surface of a plant. Often the hairs are white, giving the surface a silvery appearance. Latin="cushion stuffing" Tomentose - of a surface covered with tomentum. Tomentous - of a surface covered with tomentum. Tomentosum - epiphet of a plant covered with tomentum.

Tracheid - a basic primitive unit of the xylem (fluid-conducting tissue), consisting of a single elongated cell with pointed ends and a thick, pitted secondary cell wall composed of cellulose thickened with lignin. The mature tracheid is dead and empty of cytoplasm. Tracheids are found in all vascular plants and are the only conducting elements in gymnosperms and ferns.

Tuber - a thick, fleshy underground modified root or stem section used for storage of food or water, as in a potato.

Tubercle - small, rounded prominence or knob-like protusion from the surface. In Cactaceae a roughly pyrimidal protrusion from the stem surface, with an areole at or near its summit. Hence: adj. tuberculate.